Sperms and eggs are fertilized in vitro in a culture system to prepare fertilized eggs (zygotes) and the fertilized eggs can be further cultured until they are developed into cleavage, morula and blastocyst stages and hatched from zona pellucida into the hatched blastocyst stage. A technique of implanting a fertilized egg in the cleavage to blastocyst stage into the uterus to obtain an offspring, i.e., assisted reproductive technology (ART) has been established not only in the field of livestock but also in medical treatment for human infertility.
However, the rate of successful pregnancy by in vitro fertilization is not always high. For example, in human, the rate of successful pregnancy still remains at about 25 to 35%. As a cause of this, it is pointed out that the probability of obtaining a good fertilized egg suitable for implantation to the uterus by culture is not high. The fertilized eggs obtained by culture are individually subjected to microscopic observation by an expert to determine whether the fertilized eggs of good quality are suitable for uterus implantation.
In the in vitro fertilization, a micro-drop method, in which a drop of culture medium is prepared in a vessel, and a fertilized egg is introduced in the drop and subjected to in vitro culture, is frequently used. In the micro-drop method known in the art, a petri dish of 30 to 60 mm in diameter having a uniform flat bottom surface is used as a cell culture vessel. On the bottom surface of the petri dish, a plurality of drops of culture medium are prepared at intervals and cells are cultured in the drops. Such a cell culture method has been used.
When a drop is prepared in a petri dish conventionally used, the position of a fertilized egg changes depending upon the cellular motion of the fertilized egg itself and convection within the drop. Thus, it is difficult to identify the fertilized egg cultured in the drop and monitored. Because of the problem, it has been desired to develop means for controlling the position of a fertilized egg.
To more efficiently obtain the culture effect of a fertilized egg, it is preferable to use interaction of fertilized eggs to each other (paracrine effect). For controlling the position of a fertilized egg while using the effect, a system in which microwells having the same size as that of the fertilized egg are formed in the bottom surface of a petri dish and drop of culture medium is added so as to cover the microwells, and then fertilized eggs are disposed in microwells filled with the culture medium and cultured is known. Owing to the system, a plurality of fertilized eggs can be cultured in a small amount of culture medium while successfully controlling the positions of a plurality of fertilized eggs so as to enable monitoring of individual eggs, and the paracrine effect can be used.
To distinguish individual fertilized eggs, individual microwells must be distinguished. Since microwells are observed by a microscope, a lot of information must be estimated only from a microscope field; however, at a high magnification, which microwell is observed cannot be determined. To distinguish microwells, numerical or literal information tags are disposed to the outermost periphery of a microwell array. In this way, it is known that the microwells are distinguished with the help of a matrix system. However, this method had a problem in operability because when observation is made under a microscope at high magnification, it is necessary to read identification information by shifting the viewing position far away from the microwell. In addition, when cells are photographed by a microscope, the identification information of the microwell is not present in the photograph and thus the information must be manually provided to the photo data. Because of this, the operation was intricate and there was a risk of mistakenly associating information by the operator.